This invention relates to photoflash lamps and, more particularly, to flashlamps of the type containing primer material ignited by a high voltage pulse.
Such flashlamps typically comprise a tubular glass envelope constricted and tipped off at one end and closed at the other end by a press seal. A pair of lead-in wires pass through the glass press and terminate in an ignition structure including a glass bead, one or more glass sleeves, or a glass resevoir of some type. A mass of primer material contained on the bead, sleeve, or resevoir bridges across and contacts the ends of the lead-in wires. Also disposed within the lamp envelope is a quantity of filamentary metalic combustible, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at an initial fill pressure of several atmospheres.
An improved ignition structure, which provides excellent lamp reliability and substantial economies and ease of automated manufacture, is the so-called fritted lead construction described in U.S. Pat. No. 4,059,389, of Donald E. Armstrong et al. This is a beadless ignition structure comprising a pair of spaced-apart lead-in wires, and a coating of primer material over the frit-coated terminations. The primer may bridge the wire terminations or comprise separate spaced-apart coatings on the respective terminations with the filamentary combustible (shredded foil) being in contact with both terminations to provide a conducting path therebetween.
Although the aforementioned fritted lead construction provides a number of advantages, there are problem areas that may arise. For example, if the primer is bridged from lead to lead, expansion differentials from heating to cooling during manufacture sometimes cause the primer bridge to crack open sufficiently so that the high voltage pulse will not jump the gap to flash the lamp. If the lamp is made with separate primer-coated leads, it is necessary that the combustible shreds within the lamp contact both primer-coated lead ends to complete the circuit path. In actual practice the combustible distribution may occasionally be wadded and located such that it does not make good contact with the primer-coated lead ends and, thus, result in a lamp that fails to flash. To overcome these problems, copending application Ser. No. 971,775, filed Dec. 21, 1978, now U.S. Pat. No. 4,270,897, issued June 2, 1981, and assigned to the present assignee, describes an ignition structure comprising a pair of spaced apart metal lead-in wires sealed in one end of the glass envelope of a lamp, with the glass-sealed wires extending inside the envelope and the metal terminations of each wire within the envelope being bare of the sealing glass. In a preferred embodiment, the envelope glass protrudes inside the envelope and the lead-in wires are sealed within this protruding glass portion in a predetermined spaced-apart relationship, the protruding glass having a stretched configuration from the lead-in wires having been pushed through the end of the envelope during heat sealing. Preferably, each of the lead-in wire terminations has the configuration of a transverse cut having a substantially flat end surface, with each of the flat end surfaces being bare of glass and substantially flush with the surrounding glass surfaces. The primer material is then coated about the inner end of the protruding portion of glass within the envelope so as to cover and bridge the terminations. This construction provides a supported circuit path from lead to lead which will not crack apart due to the solid glass coating covering and supporting both internal leads; further, this construction is independent of combustible contact to complete the flashing circuit.
This pushed-through-lead structure also provides a comparatively higher ignition breakdown voltage. Although a uniformly high breakdown voltage is generally a desirable feature, this type of lamp construction requires primer modification to provide flashlamp array electrical breakdown voltages that do not exceed that provided by the minimum output of cameras on the market, e.g. about 2,000 volts. Capacitance inherent in multilamp photoflash arrays, such as that described in U.S. Pat. No. 4,164,007, reduces the actual voltage applied to the lamps therein to a value as low as 1,800 volts. Lamps with a firing or breakdown voltage above that available will fail to flash. Typical primer material formulations as used in more conventional high voltage flashlamps, e.g., such as the primer compositions described in U.S. Pat. No. 4,059,388 and copending applications Ser. No. 744,540, filed Nov. 24, 1976, and Ser. No. 865,987, filed Dec. 30, 1977, now U.S. Pat. No. 4,190,413 issued Feb. 26, 1980 both assigned to the present assignee, will, when used in lamps such as the pushed-through-lead type, give firing voltages that are too high for reliable operation.
With respect to U.S. Pat. No. 4,059,389 a primer composition is described which comprises about 99.0% by weight zirconium powder and 1.0% by weight cellulose nitrate on a dry basis. In application Ser. No. 744,540, an additive of finely divided inert material, such as fumed silica, fumed alumina or fumed titania, is included in the mixture to enhance the sensitivity and reliability of the primer material. In application Ser. No. 865,987, an additive of relatively coarse electrically nonconducting inert particulate material, such as glass microbeads, is included in the primer mixture to elevate both the average and low limit of the primer breakdown voltage.